The prior art related to the present invention is disclosed in Japanese Application Patent Announcement Publication No. Hei 08-501156 as shown in FIG. 23, for example. This Patent describes the pressure sensor manufactured according to the sacrificial layer etching technique. Sacrificial layer etching is formed in the following process, for example: A sacrificial layer to be removed later is formed on the substrate in advance, and part of this layer is removed. A film remaining as a structure or anchor is formed thereon, and the end portion of the sacrificial layer is exposed to the outside. This portion is removed by etching, then a sensor and actuator are manufactured  with the structural film. Or this process is repeated several times to form a more complicate structure. A fixed electrode 3 is formed on the surface of a silicon substrate 1 and a polysilicon diaphragm 6 is formed thereon, with a gap 7 located in-between. This gap 7 is formed by etching and removing the sacrificial layer already formed in this area through an etch channel 12 provided on part of polysilicon diaphragm 6. To close this etch channel 12 and to provide vacuum sealing of the gap 7, silicon oxide film 8 is formed to cover the whole surface of the polysilicon diaphragm 6 and part of the silicon substrate 1 are covered. As a result, gap 7 is formed as a vacuum sealed pressure reference chamber, and a capacitor is formed between the fixed electrode 3 provided on the substrate in the pressure reference chamber and a conductive diaphragm (movable electrode) consisting of the polysilicon film 6. If there is a change in the external pressure, polysilicon film is displaced by differential pressure from the pressure reference chamber and a gap is changed between the two electrodes to cause a change in the capacitance of the capacitor. This change in capacitance is used to detect the pressure.
Another prior art related to the present invention  is disclosed in the Japanese Application Patent Laid-Open Publication No. Hei 11-14482. This Patent also refers to the capacitance pressure sensor manufactured according to the sacrificial layer etching technique. In this case, a silicon nitride film is used to seal the etch channel.
To ensure reliability in the long-term use of the pressure sensor of the above-mentioned structure, it is necessary to reinforce the hermetic structure of the pressure reference chamber and to prevent temporal change of the output. This requires careful selection of an adequate sealing structure of the etch channel and a proper sealing material. Japanese Application Patent Announcement Publication No. Hei 08-501156 discloses a silicon oxide film used as a sealing material. However, silicon oxide film is permeable to moisture to some extent. In a highly humid environment, therefore, moisture may enter the gap through oxide film, causing changes in characteristics.
If the etch channel is sealed by nitride silicon film as disclosed in Japanese Application Patent Laid-Open Publication No. Hei 11-14482, characteristics of such a structure as diaphragm will be changed with time since silicon nitride film has a very great film stress after film formation. Accordingly, prevention  of deformation requires reduction in the thickness of silicon nitride film used for sealing. This will result in restrictions on the size of the etch channel, etching failure of the sacrificial layer or increased etching time.